When the Rem 870 is mentioned, the model should be discussed.
The Rem 870 Wingmaster is a better gun than the Express. Better parts and better finish (it's not just a rougher exterior). The Express was designed to compete against the lower priced Mossberg 500.
Of the Mossbergs, I prefer the 590. As mentioned by another, since its a defensive shotgun, it's heavier duty (mil spec methinks).
Of the two, I prefer the 870 Wingmaster. I'm more familiar with it and have the tools to service it.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!